Thermoplastic foams made from styrenic polymers such as polystyrene have found extensive use, particularly in food packaging and food service applications. The key to a successful polystyrene foam for food applications is to use a blowing agent composition during the formation of the foam that diffuses out of the cells and is substantially replaced by air before the foam comes into contact with any food to be packaged or served.
Generally, polystyrene foams are manufactured by mixing a volatile blowing agent with the styrenic resin under a controlled temperature and pressure sufficient to plasticize the resin and to maintain the resin blowing agent composition in an unfoamed state. Thereafter the molten mixture of resin and blowing agent and sometimes a nucleator is extruded through an annular die into a zone of lower temperature and pressure. If extrusion conditions are optimum, a tube of rigid, closed cell foam will be produced.
This tube is usually stretched over a mandrel of larger diameter. Stretching not only yields a larger tube of foam but also "orients" and strengthens or toughens the foam.
The tube is then slit and opened up to form at least one flat sheet. The sheet (or sheets) is usually stored in large rolls and aged for at least 24 hours. The aging process is required to obtain "post expansion" during the subsequent thermoforming operation.
"Post expansion" refers to the swelling of the foam as it is heated in the thermoformer oven. This expansion is the result of the different permeation rates of air and the blowing agent through the foam cell walls. As the foam emerges from the die during the extrusion step, the cells containing the blowing agent tend to expand until the pressure within the cells equals the atmospheric pressure outside the cells. During the aging period, air permeates rapidly into the cells; but the blowing agent (having a larger size molecule than air) permeates out relatively slowly. The result is an increase of cell gas pressure during aging. The gas pressure increases from 1 atmosphere to about 2 atmospheres. When the foam is subsequently heated and softened, this increased gas pressure causes the foamed product to expand further, i.e., "post expand".
Dichlorodifluoromethane (CFC-12) historically has been the blowing agent of choice in producing polystyrene foam. With the planned phase-out of CFC-12 as a foam blowing agent because of its measurable undesirable Ozone Depletion Potential (ODP) and the unacceptability of chlorodifluoromethane (HCFC-22) as a long term alternative blowing agent in food packaging/food service use, there is a critical need for an acceptable alternative blowing agent. Hydrocarbon blowing agents, although having zero ODPs, are less desirable for use in food containers since they are classified as photochemically reductive volatile organic compounds (VOCs) and their use is regulated by law.
It is an object of this invention to provide a blowing agent for thermoplastic polymers such as polyethylene or polystyrene or the like that displays a zero ODP and is not substantially photochemically reactive, that can be processed in a manner such that it diffuses substantially completely from the ultimate foamed product, and that, even if a slight amount remained in the product, it would be so low as not to be considered a component of any food served or contained within the foamed product.
It is a further object to provide an operable process for utilizing the aforementioned blowing agent in the manufacture of a polymeric foam product, particularly in the manufacture of a foam of a thermoplastic composition such as polyethylene or polystyrene or the like.